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            <h2 class="title"><a id="cam"></a>Chapter 10. 
		Berkeley DB Concurrent Data Store Applications
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          <b>Table of Contents</b>
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          <dt>
            <span class="sect1">
              <a href="cam.html#cam_intro">Concurrent Data Store introduction</a>
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            <span class="sect1">
              <a href="cam_fail.html">Handling failure in Data Store and Concurrent Data Store applications</a>
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            <span class="sect1">
              <a href="cam_app.html">Architecting Data Store and Concurrent Data Store applications</a>
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              <h2 class="title" style="clear: both"><a id="cam_intro"></a>Concurrent Data Store introduction</h2>
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        <p>It is often desirable to have concurrent read-write access to a database
when there is no need for full recoverability or transaction semantics.
For this class of applications, Berkeley DB provides interfaces supporting
deadlock-free, multiple-reader/single writer access to the database.
This means that at any instant in time, there may be either multiple
readers accessing data or a single writer modifying data.  The
application is entirely unaware of which is happening, and Berkeley DB
implements the necessary locking and blocking to ensure this behavior.</p>
        <p>To create Berkeley DB Concurrent Data Store applications, you must first initialize an environment
by calling <a href="../api_reference/C/envopen.html" class="olink">DB_ENV-&gt;open()</a>. You must specify the <a href="../api_reference/C/envopen.html#envopen_DB_INIT_CDB" class="olink">DB_INIT_CDB</a> and <a href="../api_reference/C/envopen.html#envopen_DB_INIT_MPOOL" class="olink">DB_INIT_MPOOL</a> flags
flags to that method.  It is an error to
specify any of the other <a href="../api_reference/C/envopen.html" class="olink">DB_ENV-&gt;open()</a> subsystem or recovery
configuration flags, for example, <a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOCK" class="olink">DB_INIT_LOCK</a>, <a href="../api_reference/C/envopen.html#envopen_DB_INIT_TXN" class="olink">DB_INIT_TXN</a> or <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER" class="olink">DB_RECOVER</a>
All databases must, of
course, be created in this environment by using the <a href="../api_reference/C/dbcreate.html" class="olink">db_create()</a>
function or <a href="../api_reference/CXX/db.html" class="olink">Db</a> constructor, and specifying the environment
as an argument.</p>
        <p>Berkeley DB performs appropriate locking so that safe enforcement of the
deadlock-free, multiple-reader/single-writer semantic is transparent to
the application.  However, a basic understanding of Berkeley DB Concurrent Data Store locking
behavior is helpful when writing Berkeley DB Concurrent Data Store applications.</p>
        <p>Berkeley DB Concurrent Data Store
avoids deadlocks without the need for a deadlock detector by performing
all locking on an entire database at once (or on an entire environment
in the case of the <a href="../api_reference/C/envset_flags.html#set_flags_DB_CDB_ALLDB" class="olink">DB_CDB_ALLDB</a> flag), and by ensuring that at
any given time only one thread of control is allowed to simultaneously
hold a read (shared) lock and attempt to acquire a write (exclusive)
lock.</p>
        <p>All open Berkeley DB cursors hold a read lock, which serves as a guarantee
that the database will not change beneath them;  likewise, all
non-cursor <a href="../api_reference/C/dbget.html" class="olink">DB-&gt;get()</a> operations temporarily acquire and release
a read lock that is held during the actual traversal of the database.
Because read locks will not conflict with each other, any number of
cursors in any number of threads of control may be open simultaneously,
and any number of <a href="../api_reference/C/dbget.html" class="olink">DB-&gt;get()</a> operations may be concurrently in
progress.</p>
        <p>To enforce the rule that only one thread of control at a time can
attempt to upgrade a read lock to a write lock, however, Berkeley DB must
forbid multiple cursors from attempting to write concurrently.  This is
done using the <a href="../api_reference/C/dbcursor.html#cursor_DB_WRITECURSOR" class="olink">DB_WRITECURSOR</a> flag to the <a href="../api_reference/C/dbcursor.html" class="olink">DB-&gt;cursor()</a> method.
This is the only difference between access method calls in Berkeley DB Concurrent Data Store and
in the other Berkeley DB products.  The <a href="../api_reference/C/dbcursor.html#cursor_DB_WRITECURSOR" class="olink">DB_WRITECURSOR</a> flag causes the
newly created cursor to be a "write" cursor; that is, a cursor capable
of performing writes as well as reads.  Only cursors thus created are
permitted to perform write operations (either deletes or puts), and only
one such cursor can exist at any given time.</p>
        <p>Any attempt to create a second write cursor or to perform a non-cursor
write operation while a write cursor is open will block until that write
cursor is closed.  Read cursors may open and perform reads without blocking
while a write cursor is extant.  However, any attempts to actually perform
a write, either using the write cursor or directly using the
<a href="../api_reference/C/dbput.html" class="olink">DB-&gt;put()</a> or <a href="../api_reference/C/dbdel.html" class="olink">DB-&gt;del()</a> methods, will block until all read cursors
are closed.  This is how the multiple-reader/single-writer semantic is
enforced, and prevents reads from seeing an inconsistent database state
that may be an intermediate stage of a write operation.</p>
        <p>By default, Berkeley DB Concurrent Data Store does locking on a per-database basis.  For this
reason, using cursors to access multiple databases in different orders
in different threads or processes, or leaving cursors open on one
database while accessing another database, can cause an application to
hang.  If this behavior is a requirement for the application, Berkeley DB
should be configured to do locking on an environment-wide basis.  See
the <a href="../api_reference/C/envset_flags.html#set_flags_DB_CDB_ALLDB" class="olink">DB_CDB_ALLDB</a> flag of the <a href="../api_reference/C/envset_flags.html" class="olink">DB_ENV-&gt;set_flags()</a> method
for more information.</p>
        <p>With these behaviors, Berkeley DB can guarantee deadlock-free concurrent
database access, so that multiple threads of control are free to perform
reads and writes without needing to handle synchronization themselves
or having to run a deadlock detector.  Berkeley DB has no direct knowledge of
which cursors belong to which threads, so some care must be taken to
ensure that applications do not inadvertently block themselves, causing
the application to hang and be unable to proceed.</p>
        <p>As a consequence of the Berkeley DB Concurrent Data Store locking model, the following sequences
of operations will cause a thread to block itself indefinitely:</p>
        <div class="orderedlist">
          <ol type="1">
            <li>Keeping a cursor open while issuing a <a href="../api_reference/C/dbput.html" class="olink">DB-&gt;put()</a> or <a href="../api_reference/C/dbdel.html" class="olink">DB-&gt;del()</a>
access method call.</li>
            <li>Attempting to open a write cursor while another cursor is already being
held open by the same thread of control.  Note that it is correct
operation for one thread of control to attempt to open a write cursor
or to perform a non-cursor write (<a href="../api_reference/C/dbput.html" class="olink">DB-&gt;put()</a> or <a href="../api_reference/C/dbdel.html" class="olink">DB-&gt;del()</a>)
while a write cursor is already active in another thread.  It is only a
problem if these things are done within a single thread of control --
in which case that thread will block and never be able to release the
lock that is blocking it.</li>
            <li>Not testing Berkeley DB error return codes (if any cursor operation returns
an unexpected error, that cursor must still be closed).</li>
          </ol>
        </div>
        <p>If the application needs to open multiple cursors in a single thread to
perform an operation, it can indicate to Berkeley DB that the cursor locks
should not block each other by creating a Berkeley DB Concurrent Data Store <span class="bold"><strong>group</strong></span>, using
<a href="../api_reference/C/envcdsgroup_begin.html" class="olink">DB_ENV-&gt;cdsgroup_begin()</a>.  This creates a locker ID that is shared by all
cursors opened in the group.</p>
        <p>Berkeley DB Concurrent Data Store groups use a <a href="../api_reference/C/txn.html" class="olink">TXN</a> handle to indicate the shared locker
ID to Berkeley DB calls, and call <a href="../api_reference/C/txncommit.html" class="olink">DB_TXN-&gt;commit()</a> to end the group.  This
is a convenient way to pass the locked ID to the calls where it is
needed, but should not be confused with the real transactional semantics
provided by Berkeley DB Transactional Data Store.  In particular, Berkeley DB Concurrent Data Store groups do not provide any
abort or recovery facilities, and have no impact on durability of
operations.</p>
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